Abstract.

The study of carbohydrate metabolism in perennial ryegrass (Lolium perenne L. cv. Bravo) during the first 48 h of regrowth showed that fructans from elongating leaf bases were hydrolysed first whereas fructans in mature leaf sheaths were degraded only after a lag of 1.5 h. In elongating leaf bases, the decline in fructan content occurred not only in the differentiation zone (30–60 mm from the leaf base), but also in the growth zone. Unlike other soluble carbohydrates, the net deposition rate of fructose remained positive and even rose during the first day following defoliation. The activity of fructan exohydrolase (FEH; EC 3.2.1.80) was maximal in the differentiation zone before defoliation and increased in all segments, but peaked in the growth zone after defoliation. These data strongly indicate that fructans stored in the leaf growth zone were hydrolysed and recycled in that zone to sustain the refoliation immediately after defoliation. Despite the depletion of carbohydrates, leaves of defoliated plants elongated at a significantly higher rate than those of undefoliated plants, during the first 10 h of regrowth. This can be partly attributed to the transient increase in water and nitrate deposition rate. The results are discussed in relation to defoliation tolerance.